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What happens to sewage at you local sewage treatment works?

Monday, April 15th, 2013

Many people don’t give a second thought to the sewage which leaves their homes, but the process of transporting it to sewage works and treating it is quite complex.

All the sewage or waste water which leaves your home needs to be treated before it is put safely back into rivers. This waste travels via sewage pipes to the local sewage works. In some areas there are also separate surface water sewers which channel the surface water i.e. water from roofs, roads and pavements, straight back into a river. This is why it’s important that you never pour waste of any kind into a surface drain.

Although there are networks which feature separate surface water and foul water sewers most use a combined system which transports the waste and rainwater to a sewage treatment plant. And in the city of London this is the process used.

Any water which leaves your home when you empty the sink or flush the toilet enters the sewer network. It is then pumped to the local sewage treatment works where it is treated before it is allowed to be released into a river.

When the waste water arrives at the sewage treatment plant the first cleaning stage is screening. This is where the large objects which may block the further processes are removed. Things which should never be put into the drainage system are usually removed at this stage such as nappies, sanitary items and even things such as rags and bottles.

The first stage of the treatment process is called primary treatment. This is where all the organic solids are removed from the waste water. Putting the waste into large settlement tanks allows the solids, or sludge, to sink to the bottom of the tank. During this process the sludge is pushed towards the tank centre where it pumped to a separate treatment area.

In the next part of the treatment process, called secondary treatment, the smaller solid particles are removed and the waste is aerated in aeration lanes. This encourages the growth of good bacteria which breaks down any nasty bugs in the water.

The aerated wastewater is then passed through a further settlement tank where the good bacteria sinks to the bottom to form more sludge which is returned to the secondary treatment stage of the process. The water is now clean and is allowed to pass over the settlement tank wall and directed back into the local river network. Some sewage plants also have further filtration systems in place where water is passed through a bed of sand before being allowed to return to the river.

Any sludge which is removed at the secondary treatment stage is recycled and used as fertiliser on agricultural land. However new developments also mean that this sludge can also be used as a source of energy. This energy is created by either anaerobic digestion which creates biogas or biomethane or by thermal destruction where the sludge is allowed to dry and is then burned to generate heat.